Shaped like an upturned boat, drumlin hills are found clustered together in their hundreds and thousands in distinct fields called swarms. They're the most common landform across large areas of northern North America and Europe, marking the footprint of massive ice sheets that formed during past ice ages. (Submitted photo)

It’s a mystery that has stumped geologists for more than a
century.

Now, thanks to new technology – including satellite laser
imagery – researchers may be one step closer to understanding
the origins of an archetypal landform: the drumlin hill.

“Drumlin hills are the most studied and yet the most
enigmatic ice age landform,” says U of T Scarborough geology
professor Nick
Eyles. “Thanks to high resolution satellite imagery and new
technology like LiDAR, we’re literally seeing the surface of the
planet for the first time and finding major surprises in the
process.”

Shaped like an upturned boat, drumlin hills are found clustered
together in their hundreds and thousands in distinct fields called
swarms. They are the most common landform across large areas of
northern North America and Europe, marking the footprint of massive
ice sheets that formed during past ice ages.

The question that’s stumped geologists since drumlins were
first studied more than 150 years ago is whether they were built up
progressively or sculpted out of older sediment. Eyles and his team
including PhD candidate Shane Sookhan and undergraduate student Lina
Arbelaez-Moreno were able to determine that drumlins are simply
streamlined “islands” of sediment that are often bisected
to form kilometre-long skinny megaridges. Their research, published in
the journal Sedimentary Geology, suggests that drumlins and
megaridges are all part of a single family of landforms formed by
erosion.

“The new data we were able to obtain shows that these
landforms occur on hard rock, which stresses the importance of
sculpting below the base of the ice sheet,” says
Arbelaez-Moreno.

To illustrate the importance of megaridges Eyles points to research
being done on the modern Greenland and Antarctica ice sheets. These
are slow moving ice sheets but contain faster flowing corridors called
‘ice streams’ that are up to tens of kilometres wide,
hundreds of kilometres in length and can move as fast as 1 km
annually.

“They’re essentially arteries moving huge volumes of
ice toward the margin of the ice sheet,” explains Eyles. The
thinning and retreating of modern ice streams in a warming world has
exposed their underlying beds which are seen to be megaridged, and
that appears to allow the ice to flow faster across its bed by
creating a slippery low-friction surface, he adds.

The last Canadian ice sheet (Laurentide) was as much as 3 km thick
and behaved in exactly the same way, says Eyles. “The transition
from drumlins to megaridges may record the final gasp of the ice sheet
as it warmed up and began to stream over its bed,” says
Eyles.

Debris that is being dragged under these streams is highly erosive
– “think of sandpaper’’ says Sookhan –
and the process sculpts the underlying surface, allowing drumlins to
be progressively whittled down into longer and longer megaridges.

The data used by the researchers relied on high resolution
satellite imagery and new technologies including LiDAR, which uses
hundreds of laser beams fired from planes onto the land below. The
result is the creation of highly accurate topographic maps even where
landscapes are covered by trees or water.

“We still have a lot to learn about how drumlins are formed,
but this imaging technique has changed the science by providing a new
way of looking at glacial landscapes,” says Sookhan.

The megaridges identified by Eyles and his team are particularly
common around Peterborough, Ontario at the site of one of the most
easily accessible drumlin fields in Canada.

“You could say drumlins are quintessentially Canadian,”
says Eyles. “They do occur in Europe, but are far more common
here because almost the entire country was covered by the Laurentide
Ice Sheet during the last ice age.”

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